1. Field of the Invention
This invention relates to planographic printing plates and, more particularly, to plates having a mask layer capable of being selectively removed by a laser beam to form the pattern desired to be printed.
2. Description of the Prior Art
Lithographic printing, frequently referred to as offset printing, occupies a substantial segment of the printing plate market, primarily because it is an economical method for producing a large number of copies. Most lithographic plates today are of the presensitized type. Such plates are provided with a photosensitive coating which permits the formation of an image on the plate by exposure through a master transparency and subsequent development.
It has recently been proposed, see U.S. Pat. No. 3,664,737 granted May 23, l972 for "Printing Plate Recording by Direct Exposure"(Lipp), to directly record information on a printing plate by means of a laser beam having a wave length in the actinic (UV) region. There are two major advantages of imaging by a laser beam. The first is that it permits the elimination of the master transparency. The images can be either computer generated or can be provided by scanning a paste-up or other original by appropriate photoelectronic means which in turn modulates the laser beam. The second advantage is that the signal, however generated, for modulating the laser which writes the image on the plate can be transmitted over great distances to a multiplicity of writing lasers. This obviously would be of particular significance to newspaper and magazine publishers who operate a number of regional printing facilities.
While the laser is a promising tool for the production of planographic printing plates and the proposal to directly image a presensitized lithographic printing plate with a laser beam having a wave length in the actinic region has great appeal, the proposal is not commercially practical for the reason that such lasers are extremely expensive, are not generally commercially available and, to date, their power output has been low. There are, on the other hand, non-actinic lasers available which are relatively inexpensive and which have a useful power output.
Therefore an object of this invention is to utilize non-actinic laser beams in the imaging of conventional presensitized planographic printing plates.